1 Technical Field of the Invention
The present invention relates generally to a transmission system working to transmit data on a frame basis, and more particularly to an improved structure of such a system designed to provide lowered operation load and power consumption and a receiver which may be employed in the transmission system.
2 Background Art
As one of data transmitting techniques, there is known a transmission system which works to transmit data on a frame basis and is in widespread use in various wire and wireless data transmission applications.
For instance, automotive keyless entry systems are used which actuate a door lock mechanism of a vehicle from a remote location using a wireless remote controller to lock or unlock doors of the vehicle. Keyless entry systems of such a type typically work to frame and output command data on door locking or unlocking from a portable transmitter to a receiver which is installed in the vehicle and works to decode the data in the received frame. An electronic control unit installed in the vehicle receives the decoded data from the receiver and actuates the door lock mechanism.
The receiver installed in the vehicle is usually actuated by power supplied from a storage battery mounted in the vehicle and adapted to be turned on intermittently in order to minimize a decrease in available capacity in the battery. Accordingly, the time at which the receiver is turned on in a cycle may not match the time at which data is outputted from the transmitter. If such an event occurs, it may cause the receiver to receive an output from the transmitter from the middle thereof.
Even if the receiver has received a frame outputted from the transmitter completely during the on-mode thereof, electromagnetic noises may result in data emission from the frame, thus leading to a malfunction of the system.
In order to avoid the above problem, typical keyless entry systems are adapted to transmit a plurality of same frames (e.g., three same frames) in sequence on each one-time actuation of a switch of the transmitter and, when the receiver has received one of the frames completely, executes a task provided by data in the frame. The system, however, continues to decode data as long as frames are received subsequently. This is because data of frames outputted in sequence from the transmitter may be altered, for example, in a case where an operator re-depresses a door-unlocking switch on the portable transmitter just after depressing a door-locking switch. Moreover, in a case where the system is designed to perform a function to close windows of the doors when the door-locking switch is held depressed for a time, it is necessary to monitor whether same frames have been outputted in sequence from the transmitter for a given period of time or not. The receiver, thus, decodes data each time it receives one of the series of frames. This may, therefore, result in increases in operation load and power consumption of the system or impinge upon processing of another task.